iPhone 6s replace battery

Contact an Apple store or Apple authorized service center to do that.

Make an appointment at an Apple store genius bar, or use the Contact Support link at the top of this page for mail-in service. The cost of a battery replacement in the US is $49. Prices are comparable in other countries.

But first make sure the problem is the battery. Go to Settings/Battery - Battery Health. If it says the battery is not up to standards or the percentage is less than 80% then you need a new battery. If not then you need to look for other possible causes.